Verifying User at Host

Hi:

For a given form, the email input field is so important for me that I would like to make sure that the informed User at Host really exists. Is there a way of verifying this in a Perl Script and return a warning message to the user instead of sending the form?

Who is Participating?

The only way to find out if a given email address exists is to send an email to it with some form of identification (such as a randomly generated 15 character sequence), and have the user respond to it (either by hitting reply and send, or following a link to a webpage (with the sequence in the query string).

Some SMTP servers allow you to ask if a user exists, but not all do, so you can't depend on it.